A conventional injector drive device (for example JP-2005-16431A) includes a power supply unit such as a DC-DC converter, and a drive switch unit (injector drive unit) that is supplied with power from the power supply unit and drives an injector and the like. In the power supply unit, a voltage (12V or 24V) supplied from a battery is raised or boosted to a high voltage of tens to hundreds of volt and is supplied to the injector drive unit.
In recent years, it is required to finely control fuel injection to reduce noxious substances contained in emission gas. To cope with this, it is required to increase the number of times of fuel injection by injector operation. If the number of times of injector operation is increased, energy for driving an injector is correspondingly increased. Therefore, it is required to increase the power output capacity of the power supply unit, and this increases the amount of heat produced in the power supply unit.
The power supply unit is usually housed in a housing of the drive switch unit. Heat produced in the power supply unit or an injector drive unit is radiated to the atmosphere through the housing. When the amount of heat produced in the power supply unit is increased, it is required to increase the size of the housing in correspondence to increase in the amount of produced heat, thereby ensuring a required heat radiation capability. Therefore, when the capacity of the power supply unit is increased to increase the number of times of injector operation, the housing, that is, the drive switch unit, need be sufficiently large.